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Abstract:

Provided is a stainless-steel seamless belt formed by subjecting a
stainless-steel plate to a plastic forming process and having a thickness
of 50 μm or less. The seamless belt has a bending durability of
100,000 or more times in a specific bending test and has a Vickers
hardness of 450 (Hv) or more.

Claims:

1. A stainless-steel seamless belt which is formed by subjecting a
stainless-steel plate to a plastic forming process and has a thickness of
50 μm or less, wherein: in a bending test in which a strip-like
stainless-steel test specimen with a width of 20 mm and a length of 45
mm, formed by cutting the seamless belt into a width of 20 mm along a
peripheral direction thereof and cutting open in a direction
perpendicular to the peripheral direction, is bent by causing a side
support plate having a rotating shaft on one side and rotating around the
rotating shaft to repeatedly reciprocate from a top dead point having an
opening distance of 29 mm to a bottom dead point having an opening
distance of 5.5 mm at a speed of 400 rpm, the strip-like stainless-steel
test specimen has a bending durability of 100,000 or more times, and
wherein: a Vickers hardness of the seamless belt is 450 (Hv) or more.

2. A method of manufacturing the stainless-steel seamless belt according
to claim 1, comprising: (1) obtaining a cup-shaped member by subjecting a
stainless-steel plate to a drawing process; (2) obtaining a thin-walled
sleeve by subjecting the cup-shaped member to a plastic forming process
to be thin-walled and cutting a base portion of the thin-walled
cup-shaped member; (3) inserting the thin-walled sleeve into a
cylindrical outer die having an internal diameter larger than an external
diameter of the thin-walled sleeve so as to be coaxial with the
cylindrical outer die, pressurizing the thin-walled sleeve from an inside
thereof so as to cause the thin-walled sleeve to closely contact an inner
wall of the outer die, and thereby expanding a diameter of the
thin-walled sleeve; and (4) subjecting the thin-walled sleeve whose
diameter is expanded in (3) to low temperature annealing at 200 to
400.degree. C., wherein the step (3) includes expanding the diameter of
the thin-walled sleeve without changing the length in a direction
perpendicular to the peripheral direction of the thin-walled sleeve.

3. A fixing belt comprising the seamless belt according to claim 1, an
elastic layer, and a toner releasing layer in that order.

4. A heat fixing apparatus comprising the fixing belt according to claim
3, a heater arranged in contact with an inner peripheral surface of the
fixing belt, and a pressure roller forming a fixing nip together with the
fixing belt.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a stainless-steel seamless belt
for use in a base material of a fixing belt for use in a heat fixing
apparatus of an electrophotographic image forming apparatus, and a
manufacturing method therefor. In addition, the present invention relates
to a fixing belt and a heat fixing apparatus for use in the
electrophotographic image forming apparatus.

[0005] Recent heat fixing apparatuses use a power-saving ceramic heater as
a heat source to heat a resin belt or a metal belt with a small heat
capacity. FIG. 2 schematically illustrates an example of the heat fixing
apparatus. In the heat fixing apparatus illustrated in FIG. 2, a
heat-resistant belt (fixing belt 33) is interposed between a ceramic
heater 31 as a heat source and a pressure roller 35 as a pressure member
to form a fixing nip portion 36. In the fixing nip portion 36, a
recording material P bearing an unfixed toner image T to be subjected to
heat fixing is introduced between the fixing belt 33 and the pressure
roller 35. The recording material P is conveyed with being sandwiched
between the fixing belt 33 and the pressure roller 35. Thus, in the
fixing nip portion 36, heat of the ceramic heater 31 is transmitted to
the recording material P through the fixing belt 33. The heat and the
pressure allow the unfixed toner T on the recording material P to be
fixed to the recording material P. In FIG. 2, the heat fixing apparatus
further includes a belt guide member 32 and a pressurizing rigid stay 34.

[0006] Examples of materials of the fixing belt for use in such a heat
fixing apparatus generally include a heat-resistant resin, and
particularly include a polyimide resin excellent in heat-resistance and
strength. Unfortunately, with a recent improvement in process speed, the
resin fixing belt has insufficient thermal conductivity. In other words,
it may be difficult to fix an unfixed toner image to a recording material
in a short time and in a sufficient manner. In light of this, there has
been proposed that the fixing belt is provided with a base layer
containing a metal excellent in thermal conductivity such as stainless
steel, nickel, aluminum, and copper (see Japanese Patent No. 3970122).

[0007] Japanese Patent No. 3970122 describes a method of manufacturing a
stainless base layer. Specifically, the following method is described as
a first embodiment.

[0008] First, a step is performed of obtaining a cup-shaped stainless
cylindrical member (a first-stage metal tube) by subjecting a stainless
metal flat plate to a deep-drawing process a plurality of times. Next, a
step is performed of obtaining a seamless belt (a second-stage metal
tube) by cutting a cup base portion by performing an ironing process
using a general spinning drawing process to form a stainless cylindrical
member with a predetermined thickness or by performing an ironing process
using a continuous die while rotating the first-stage metal tube.
Finally, a metal belt of a desired inverted crown shape (a third-stage
metal tube) is obtained by subjecting the second-stage metal tube to a
hydraulic bulge forming process followed by internal pressure load and
plastic forming processes.

[0009] Further, as a second embodiment, there is proposed a manufacturing
method which includes obtaining a metal belt by performing a complete
annealing process at 800° C. to 1,100° C., between the
steps of obtaining the second-stage metal tube and the third-stage metal
tube according to the first embodiment.

[0010] Furthermore, as a third embodiment, there is proposed a method
which includes obtaining a metal belt of an inverted crown shape (a
third-stage metal tube) by controlling the pressure and the speed at the
spinning drawing process in the step of obtaining the second-stage metal
tube according to the first embodiment.

SUMMARY OF THE INVENTION

[0011] According to the study made by the present inventor, the seamless
belt obtained by the first and third embodiments according to Japanese
Patent No. 3970122 is repeatedly bent in the nip portion, and the stress
has caused a metal fatigue to be accumulated, leading to damage such as
breaks and cracks. Further, as to the seamless belt obtained by the
second embodiment, the metal belt may be softened to reduce the hardness
by the complete annealing at 800° C. to 1,100° C. and worn
due to sliding with the ceramic heater. Thus, the present inventor has
confirmed his recognition that the metal seamless belt according to
Japanese Patent No. 3970122 still needs more improvement in the
durability.

[0012] In view of this, the present invention is directed to providing a
stainless-steel seamless belt which is excellent in durability and
difficult to crack and break even at the time of repeated bending, and
the manufacturing method therefor.

[0013] Further, the present invention is directed to providing a fixing
belt having high durability.

[0014] Furthermore, the present invention is directed to providing a heat
fixing apparatus which can fix an unfixed toner image to a recording
material in a stable manner.

[0015] According to one aspect of the present invention, there is provided
a stainless-steel seamless belt which is formed by subjecting a
stainless-steel plate to a plastic forming process and having a thickness
of 50 μm or less, wherein, in a bending test in which a strip-like
stainless-steel test specimen with a width of 20 mm and a length of 45
mm, formed by cutting the seamless belt into a width of 20 mm along a
peripheral direction thereof and cutting open in a direction
perpendicular to the peripheral direction, is bent by causing a side
support plate having a rotating shaft on one side and rotating around the
rotating shaft to repeatedly reciprocate from a top dead point having an
opening distance of 29 mm to a bottom dead point having an opening
distance of 5.5 mm at a speed of 400 rpm, the strip-like stainless-steel
test specimen has a bending durability of 100,000 or more times and
wherein a Vickers hardness of the seamless belt is 450 (Hv) or more.

[0016] Further, according to another aspect of the present invention,
there is provided a method of manufacturing a stainless seamless belt,
comprising: (1) obtaining a cup-shaped member by subjecting the
stainless-steel plate to a drawing process; (2) obtaining a thin-walled
sleeve by subjecting the cup-shaped member to a plastic forming process
to be thin-walled and cutting a base portion of the thin-walled
cup-shaped member; (3) inserting the thin-walled sleeve into a
cylindrical outer die having an internal diameter larger than an external
diameter of the thin-walled sleeve so as to be coaxial with the
cylindrical outer die, pressurizing the thin-walled sleeve from an inside
thereof so as to cause the thin-walled sleeve to closely contact an inner
wall of the outer die, and thereby expanding a diameter of the
thin-walled sleeve; and (4) subjecting the thin-walled sleeve whose
diameter is expanded in (3) to low temperature annealing at 200 to
400° C., wherein the step (3) includes expanding the diameter of
the thin-walled sleeve without changing the length in a direction
perpendicular to the peripheral direction of the thin-walled sleeve.

[0017] Furthermore, according to still another aspect of the present
invention, there is provided a fixing belt comprising the seamless belt,
an elastic layer, and a toner releasing layer in that order.

[0018] Still furthermore, according to the present invention, there is
provided a heat fixing apparatus comprising the fixing belt, a heater
arranged in contact with an inner peripheral surface of the fixing belt,
and a pressure roller forming a fixing nip together with the fixing belt.

[0019] Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference to the
attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1A is a before-internal-pressure-load state diagram of a
hydraulic bulge forming method used in embodiments of the present
invention. FIG. 1B is a during-internal-pressure-load state diagram of
the hydraulic bulge forming method used in the embodiments of the present
invention.

[0021]FIG. 2 is a schematic view illustrating an example of a schematic
configuration of an image heating apparatus of a heat fixing type.

[0022] FIGS. 3A, 3B, 3C and 3D are a schematic view illustrating steps of
the hydraulic bulge forming method for a seamless belt. FIG. 3A is a
cross-sectional view illustrating a state in which the seamless belt is
inserted into a bulge die of a hydraulic bulge forming apparatus. FIG. 3B
is a cross-sectional view illustrating a state in which an internal
pressure load is applied to inside the bulge inner die. FIG. 3C is a
cross-sectional view illustrating a state in which a pressurized fluid is
removed from the bulge inner die and after pressurization completes. FIG.
3D is a cross-sectional view illustrating a state in which the seamless
belt is being discharged from the hydraulic bulge forming apparatus.

[0023]FIG. 4A is a before-internal-pressure-load state diagram of a
conventional hydraulic bulge forming method. FIG. 4B is a
during-internal-pressure-load state diagram of the conventional hydraulic
bulge forming method.

[0024]FIG. 5 is a schematic view illustrating a deep-drawing process used
in the embodiments of the present invention.

[0025] FIG. 6 is a schematic view illustrating steps of ironing and
cutting used in the embodiments of the present invention.

[0026] FIGS. 7A and 7B are a schematic view illustrating a hydraulic bulge
forming method according to a second embodiment of the present invention.
FIG. 7A is a before-internal-pressure-load state diagram, and FIG. 7B is
a during-internal-pressure-load state diagram.

[0027]FIG. 8A schematically illustrates a state in which a bending tester
used in the embodiments of the present invention is at top dead point.
FIG. 8B illustrates a state in which the bending tester used in the
embodiment of the present invention is at bottom dead point.

[0028]FIG. 9 is an explanatory drawing describing positions of cutting
out a test specimen for bending durability and a test specimen for
hardness measurement of the present invention.

[0029]FIG. 10 is a partial sectional view of a fixing belt according to
the present invention.

DESCRIPTION OF THE EMBODIMENTS

[0030] Preferred embodiments of the present invention will now be
described in detail in accordance with the accompanying drawings.

[0031] The present inventor has focused attention on the finding that a
stainless steel subjected to a cold plastic forming process improves
hardness and fatigue strength through low temperature annealing.

[0032] In light of this, in order to further improve a dislocation density
of the seamless belt in a thickness direction thereof, the thin-walled
seamless belt having been subjected to a cold ironing process is further
subjected to a plastic forming process of expanding a diameter thereof so
as not to change the length in a direction perpendicular to the
peripheral direction. Subsequently, the seamless belt is subjected to low
temperature annealing in a temperature range of 200 to 400° C.
Thus, the stainless-steel seamless belt is obtained which has high
hardness and is excellent in bending-resistant fatigue characteristics.

[0033] Here, bulge forming is used as the diameter expansion method. When
the diameter of a thin-walled sleeve is expanded by the general bulge
forming, the diameter of the thin-walled sleeve is expanded while the
length in a direction perpendicular to the peripheral direction is
reduced. In contrast to this, the present invention expands the diameter
of the thin-walled sleeve without changing the length in a direction
perpendicular to the peripheral direction. Thus, a large shear stress is
applied to the thin-walled sleeve in a direction perpendicular to the
ironing direction, namely, in a direction of diameter expansion. As a
result, it is considered that the dislocation density effective in
increasing the strength of the seamless belt can be increased.

[0034] Further, following the diameter expansion by the bulge forming, low
temperature annealing is performed. Thus, the dislocation in the
thin-walled sleeve is considered to be able to be fixed in a crystal of
the stainless steel. More specifically, the seamless belt having a high
dislocation density has a high strength by itself, but when the seamless
belt is repeatedly bent, the stress due to the bending moves the
dislocation from inside to the surface of the seamless belt, which is
considered to cause cracks. Meanwhile, the low temperature annealing is
considered to be able to prevent a dislocation from easily moving even by
the bending in such a manner that a dislocation originally having a high
potential energy is fixed to a heterogeneous element or a point fault in
the stainless steel to thereby reduce the potential energy.

[0035] For the above reason, the stainless-steel seamless belt obtained by
the method of the present invention is considered to have both high
hardness and excellent bending durability as follows.

[0036] Vickers hardness of 450 (Hv) or more;

[0037] Bending durability of 100,000 or more times in a bending test in
which a strip-like stainless test specimen with a width of 20 mm and a
length of 45 mm formed by cutting the seamless belt into a width of 20 mm
along a peripheral direction thereof and cutting open the seamless belt
in a direction perpendicular to the peripheral direction is bent by
causing a side support plate having a rotating shaft on one side and
rotating around the rotating shaft to repeatedly reciprocate from a top
dead point having an opening distance of 29 mm to a bottom dead point
having an opening distance of 5.5 mm at a speed of 400 rpm.

[0038] Now, embodiments of the present invention will be described in
detail.

[0039] The stainless seamless belt according to the present invention is
formed by subjecting a stainless-steel plate to a plastic forming process
to have a thickness of 50 μm or less. Further, the stainless seamless
belt has a bending durability of 100,000 or more times in a bending test
in which a strip-like stainless test specimen with a width of 20 mm and a
length of 45 mm, formed by cutting the seamless belt along a peripheral
direction thereof and further cutting it into a width of 20 mm and
cutting open the seamless belt in a direction perpendicular to the
peripheral direction, is bent by causing a side support plate having a
rotating shaft on one side and rotating around the rotating shaft to
repeatedly reciprocate from a top dead point having an opening distance
of 29 mm to a bottom dead point having an opening distance of 5.5 mm at a
speed of 400 rpm. Furthermore, the stainless seamless belt has a Vickers
hardness of 450 (Hv) or more.

[0040] The aforementioned stainless seamless belt having high hardness and
excellent in bending resistance can be manufactured by a method including
the following steps (1) to (4).

[0041] (1) A cup-shaped member is obtained by subjecting a stainless-steel
plate to a drawing process.

[0042] (2) A thin-walled sleeve is obtained by subjecting the cup-shaped
member obtained in step (1) to a plastic forming process to be
thin-walled and cutting a base portion of the thin-walled cup-shaped
member.

[0043] (3) The thin-walled sleeve obtained in step (2) is inserted into a
cylindrical outer die having an internal diameter larger than an external
diameter of the thin-walled sleeve so as to be coaxial with the
cylindrical outer die. The thin-walled sleeve is pressurized from inside
thereof so as to cause the thin-walled sleeve to closely contact an inner
wall of the outer die. At this time, it is necessary to expand a diameter
of the thin-walled sleeve without changing the length in a direction
perpendicular to the peripheral direction of the thin-walled sleeve.

[0045] The step (3) differs from the conventional bulge forming in that
when a thin-walled sleeve arranged inside the cylindrical outer die
coaxially with the cylindrical outer die is pressurized from inside the
thin-walled sleeve to closely contact an inner wall of the cylindrical
outer die to expand the diameter, the length in a direction perpendicular
to the peripheral direction of the thin-walled sleeve, namely, the length
in the axial direction is not changed. In general bulge forming, with the
diameter expansion of the thin-walled sleeve, the length along the axial
direction of the thin-walled sleeve is reduced.

[0046] In contrast to this, the present invention is devised to prevent a
change in the length along the axial direction of the thin-walled sleeve
even in an advancement of diameter expansion of the thin-walled sleeve.
Specifically, for example, both ends of a thin-walled sleeve are clamped
by both ends of a cylindrical mold.

[0047] Thus, when the change in the length in a direction perpendicular to
the peripheral direction of the thin-walled sleeve is suppressed in the
diameter expansion step, a larger shear stress is added to the
thin-walled sleeve during diameter expansion. As a result, many lattice
defects called dislocations can be generated in a crystal of stainless
steel constituting the thin-walled sleeve. Thus, the dislocation density
in the crystal of stainless steel increases and the strength of a
thin-walled sleeve 1 increases. The dislocation at this time, however, is
considered to be easily moved when an external force such as bending is
added to the thin-walled sleeve. When repeated bending causes a
dislocation to move near the surface of a thin-walled sleeve, the
dislocation is considered to cause cracks or breaks to be generated in
the thin-walled sleeve.

[0048] In light of this, the thin-walled sleeve 1 with increased
dislocation density in the crystal is subjected to low temperature
annealing in step (4). Thus, the dislocation is fixed to a point fault in
the stainless steel. The resulting thin-walled sleeve can be used as a
seamless belt having high hardness and excellent in bending resistance.

[0049] The stainless seamless belt obtained in this manner can be used as
a fixing belt for use in a fixing belt for use in a heat fixing apparatus
of an electrophotographic image forming apparatus. FIG. 10 is a schematic
sectional view in a direction perpendicular to the peripheral direction
of a fixing belt 1000 according to the present invention. In FIG. 10, the
fixing belt 1000 according to the present invention includes a seamless
belt 1001, an elastic layer 1002, and a toner releasing layer 1003. The
elastic layer 1002 is disposed to secure a sufficient width of the fixing
nip formed between the pressure roller and the seamless belt. The fixing
nip allows an unfixed toner to be heat-fixed to a recording material
without adding excessive pressure and thus a high quality
electrophotographic image can be formed.

[0050] The material of the elastic layer 2 is not particularly limited,
but any material excellent in heat resistance and thermal conductivity
may be selected. Examples of the material of the elastic layer 2 can
include at least one material selected from the group consisting of
silicone rubber, fluororubber and fluorosilicone rubber, and particularly
silicone rubber can be used. Specific examples of the material of the
silicone rubber include polydimethylsiloxane,
polymethyltrifluoropropylsiloxane, polymethylvinylsiloxane,
polytrifluoropropylvinylsiloxane, polymethylphenylsiloxane,
polyphenylvinylsiloxane, and polysiloxane copolymer.

[0051] In order to obtain excellent fixed image quality, the thickness of
the elastic layer 1002 can be 10 μm or more and 1000 μm or less,
and further can be 50 μm or more and 500 μm or less.

[0052] Examples of the material of the toner releasing layer 1003 include
fluororesin, silicone resin, fluorosilicone rubber, fluororubber, and
silicone rubber made of PFA (tetrafluoroethylene/perfluoroalkylether
copolymer), PTFE (polytetrafluoroethylene), FEP
(tetrafluoroethylene/hexafluoropropylene copolymer), and the like. In
particular, PFA can be used because toner and the like are do not easily
adhere to it.

[0053] The fixing belt 33 used in the heat fixing apparatus illustrated in
FIG. 2 can used as the fixing belt 1000 according to the present
invention to constitute the heat fixing apparatus according to the
present invention.

[0054] The present invention can provide a stainless-steel seamless belt
that is difficult to crack and break due to repeated bending, and
excellent in durability. Further, the present invention can provide a
fixing belt excellent in durability. Furthermore, the present invention
can provide a heat fixing apparatus which can fix an unfixed toner image
to a recording material in a stable manner.

[0055] Now, a method of manufacturing a stainless-steel seamless belt
according to the present invention will be specifically described with
reference to the embodiments given below.

First Embodiment

[0056] In the present embodiment, the stainless-steel seamless belt
manufacturing method will be described in regard to separate steps from
obtaining a first-stage metal tube to obtaining a fourth-stage metal
tube.

[0057] (First-Stage Metal Tube)

[0058]FIG. 5 illustrates a method of manufacturing a first-stage metal
tube by making use of a deep-drawing process. Here, a stainless-steel
plate is subjected to a drawing process to obtain a first-stage metal
tube (a cup-shaped member 104).

[0059]FIG. 5 includes a blank 100 which is a circular stainless-steel
plate or a metal flat plate (blank) with a thickness of about 200 to 400
μm. FIG. 5 further includes a drawing punch 101 and a cylindrical
drawing die 102 which is a mold made of a metal material having a surface
subjected to super-hard plating.

[0060] In FIG. 5, the metal flat plate 100 is interposed between the
drawing punch 101 and the drawing die 102, and then the drawing punch 101
is pressed down toward the drawing die 102 in a direction indicated by
the arrow. At the same time, the metal flat plate 100 is sandwiched
between a blank holder 103 and an upper surface of the drawing die 102 to
be subjected to a drawing process so as to prevent wrinkling. Further,
lubricating oil with high viscosity or a solid lubricant such as graphite
and molybdenum disulfide is interposed between the metal flat plate 100
and the drawing punch 101 and between the metal flat plate 100 and the
drawing die 102 to improve drawability.

[0061] The above steps are repeated, generally about 2 to 6 times so as to
gradually reduce the cup diameter using different molds to perform a
deep-drawing process to manufacture a cup-shaped member 104 made of
metal. Note that the method of obtaining the metal cup-shaped member is
not limited to the present embodiment, but a cup-shaped member may be
obtained by impact molding, welding, cutting, or the like.

[0062] (Second-Stage Metal Tube)

[0063] Next, the cup-shaped member 104 is subjected to a plastic forming
process to produce a bottomed cylindrical member 2. Then, the base
portion of the bottomed cylindrical member 2 is cut away to obtain a
thin-walled sleeve 1.

[0064] FIG. 6 illustrates a series of steps in the second stage.

[0065] First, the cup-shaped member 104 manufactured in the first stage is
placed on an ironing punch 105 in a manner to cover the front end of the
ironing punch 105 as illustrated in FIG. 6. Then, an ironing die 106 is
lowered in a direction indicated by the arrows so as to sandwich the
metal cup-shaped member 104 therebetween to perform an ironing process in
a gap between the ironing punch 105 and the ironing die 106. In the
present embodiment, a plurality of ironing dies 106 with gradually
reduced internal diameters are continuously lowered along the metal
cup-shaped member 104 until a predetermined thin thickness is obtained.
Note that the ironing method is not limited to the above method. For
example, any plastic forming method capable of achieving a total ironing
rate (plate-thickness reduction rate) of 75% or more may be applied to
the ironing process for the cup-shaped member 104.

[0066] Then, the bottomed cylindrical member 2 is demolded from the
ironing punch 105. Then, the base portion of the bottomed cylindrical
member 2 is cut away by a cutting outer edge 107 and a cutting inner edge
108 to obtain the thin-walled sleeve 1.

[0067] (Third-Stage Metal Tube)

[0068] Next, the stainless-steel thin-walled sleeve 1 obtained in the
second stage is subjected to a plastic forming process to further
increase the dislocation density in the thin-walled sleeve 1. In the
present stage of the manufacturing method, the thin-walled sleeve 1
obtained in the second stage is inserted into the interior of an outer
die, pressurized from inside the thin-walled sleeve so as to cause the
thin-walled sleeve to closely contact an inner wall of the outer die to
expand the diameter of the thin-walled sleeve, namely, performing
diameter expansion.

[0069] The present embodiment uses a diameter expansion method using
hydraulic bulge forming as means of efficiently increasing the
dislocation density of the thin-walled sleeve (stainless-steel seamless
belt) subjected to the ironing process. The hydraulic bulge forming
refers to a method of molding a metal tube into a desired shape by
introducing a pressurized fluid into the metal tube to be processed to
apply a pressure (hereinafter referred to as an internal pressure).

[0070] FIGS. 3A to 3D illustrate the hydraulic bulge forming method to be
applied to the thin-walled sleeve 1 in the order of steps in the present
stage. With reference to FIGS. 3A to 3D, a hydraulic bulge forming
apparatus is illustrated which is composed of a bulge outer die 10 and a
bulge inner die 20. The bulge inner die 20 is provided with an envelope
structure of holding a rubber tube 24 with the aid of rubber tube holding
members 21a and 21b so as to prevent a pressurized fluid 26 from leaking
outside. When the pressurized fluid 26 is introduced from a pressurized
fluid passage 25, the rubber tube 24 is expanded in a direction
contacting the bulge outer die 10 to increase the diameter of the
thin-walled sleeve 1.

[0071] FIG. 3A is a cross-sectional view illustrating a state in which the
thin-walled sleeve 1 is inserted into a bulge outer die of the hydraulic
bulge forming apparatus. In the present embodiment, the thin-walled
sleeve 1 manufactured in the second stage was subjected to a cleaning
step to remove the lubricant attached during the ironing process in the
second stage. Then, the thin-walled sleeve 1 was inserted into the bulge
die.

[0072] FIG. 3B is a cross-sectional view illustrating a state in which an
internal pressure load is applied to inside the bulge inner die 20. The
rubber tube 24 is expanded by introducing a pressurized fluid through the
pressurized fluid passage 25 to press the outer wall of the thin-walled
sleeve 1 toward the inner wall of the bulge outer die 10. At this time, a
space filled with the pressurized fluid 26 is created between the rubber
tube 24 and an inner die axial center 23.

[0073] FIG. 3C is a cross-sectional view illustrating a state in which the
pressurized fluid is removed from the bulge inner die 20 and after the
pressurization step for the thin-walled sleeve 1 completes. The
thin-walled sleeve 1 is in a state of being released from the state of
closely contacting the bulge outer die 10 and is in a state of not
contacting the rubber tube 24 or the bulge outer die 10.

[0074]FIG. 3D is a cross-sectional view illustrating a state in which the
thin-walled sleeve 1 is being discharged from the hydraulic bulge forming
apparatus.

[0075] Through the above steps, the third-stage metal tube can be thus
obtained.

[0076] In the diameter expansion step applied to the thin-walled sleeve 1
illustrated in FIG. 3B, the diameter of the thin-walled sleeve 1 needs to
be expanded without changing the length in the direction perpendicular to
the peripheral direction of the thin-walled sleeve 1. By referring to
FIGS. 1 and 4, the difference between the conventional hydraulic bulge
forming method and the hydraulic bulge forming method of the present
invention will be described in detail.

[0077] FIGS. 4A and 4B are an enlarged view describing a change in the
front end portion of the stainless-steel seamless belt between a state
before an internal pressure load is applied and a state while an internal
pressure load is being applied according to the conventional hydraulic
bulge forming method. FIGS. 1A and 4B are an enlarged view illustrating a
change in the front end portion of the stainless-steel seamless belt
between a state before an internal pressure load is applied and a state
while an internal pressure load is being applied according to the
hydraulic bulge forming method of the present invention.

[0078] According to the conventional hydraulic bulge forming method
illustrated in FIGS. 4A and 4B, when an internal pressure load is applied
to expand the diameter of the thin-walled sleeve 1, the length of the
thin-walled sleeve 1 in the direction perpendicular to the peripheral
direction of the thin-walled sleeve is accordingly reduced. More
specifically, when a comparison is made between the
before-internal-pressure-load state and the after-internal-pressure-load
state in which the thin-walled sleeve 1 is in a close contact with the
bulge outer die 10, the length of the thin-walled sleeve 1 in the
direction perpendicular to the peripheral direction of the thin-walled
sleeve shrinks by H after the thin-walled sleeve 1 is brought into close
contact with the bulge outer die 10.

[0079] In contrast to this, according to the hydraulic bulge forming
method of the present invention illustrated in FIGS. 1A an 1B, a
cylindrical binding member 11 having an internal diameter smaller than an
internal diameter of the bulge outer die 10 is mounted on both ends of
the bulge outer die 10. Although not illustrated in FIGS. 1A and 1B, a
similar binding member 11 is mounted on opposite end sides of the
seamless belt 1. A frictional force between the binding member 11 and the
rubber tube 24 causes a fixing portion G to be formed in the end portions
of the thin-walled sleeve 1. The fixing portion G allows the diameter of
the thin-walled sleeve 1 to be expanded while suppressing a change in the
length of the thin-walled sleeve 1 in the direction perpendicular to the
peripheral direction of the thin-walled sleeve. In other word, as
illustrated in FIGS. 4A and 4B, the present invention can suppress a
generation of shrinkage H in the direction perpendicular to the
peripheral direction of the thin-walled sleeve 1, which shrinkage may be
due to diameter expansion.

[0080] Note that the unit for suppressing a change in the length of the
thin-walled sleeve 1 in the direction perpendicular to the peripheral
direction of the thin-walled sleeve is not limited to the present
embodiment. For example, a method may be used such that a force in a
direction opposite to that in the present embodiment, namely, a pressure
is applied from an outer surface side of the stainless-steel seamless
belt toward the axial center to fix the both end portions of the seamless
belt and thereby suppress a change in the length of the belt in the
direction perpendicular to the peripheral direction.

[0081] (Fourth-Stage Metal Tube)

[0082] Finally, in order to change movable dislocations grown by the
plastic forming to fixed dislocations considered to be effective for
bending resistance fatigue, the stainless-steel seamless belt
(thin-walled sleeve) whose diameter is expanded in the third stage is
subjected to low temperature annealing at 200 to 400° C. to obtain
a fourth-stage metal tube. In the present embodiment, the fourth-stage
metal tube was subjected to low temperature annealing for about 30
minutes in an electric furnace preliminarily heated to 300° C.

[0083] Evaluation Method

[0084] (Bending Endurance Test)

[0085] FIGS. 8A and 8B illustrate a bending test method. First, a
stainless-steel seamless belt is cut into a width of 20 mm in an axial
direction and then cut open the belt in the direction perpendicular to
the peripheral direction to obtain a 45 mm-long strip-like
stainless-steel test specimen 200. FIG. 9 illustrates positions at which
bending test specimen 91 is cut out from the stainless-steel seamless
belt. The number of bending tests tended to be small on an opening
portion side (ironing terminal portion side). Thus, the stainless-steel
seamless belt was cut at a position 20 mm in the axial direction from the
opening portion side (ironing terminal portion side) to obtain a test
specimen.

[0086]FIG. 8A is a cross sectional view illustrating a state in which a
test specimen 200 is attached to a bending endurance tester. The test
specimen 200 was sandwiched between a horizontal base 202 and a side
support plate 201 having a rotating shaft 203 on one side and rotating
around the rotating shaft. At this time, the opening distance A of the
test specimen 200 is 29 mm.

[0087]FIG. 8B is a cross sectional view illustrating a state in which the
test specimen 202 receives a maximum bending in the tester for use in the
present invention. At this time, the side support plate 201 and the
horizontal base 202 are in a horizontal state where the opening distance
B of the test specimen 200 is 5.5±0.2 mm.

[0088] In the bending tester, the side support plate 201 repeatedly
reciprocated from a top dead point with an opening distance of 29 mm to a
bottom dead point with an opening distance of 5.5 mm at a speed of 400
rpm. Then, the test specimen was visually checked for a crack to count
the number of reciprocating movements until the crack was found. Note
that the test specimen was sufficiently polished with #2000 sandpaper to
such a state that the cut end surface did not affect the bending test
results.

[0089] (Hardness Measurement)

[0090] Now, the hardness measurement method will be described. The
hardness was measured in accordance with a test method conforming to
ISO14577-1.

[0091]FIG. 9 illustrates a position of cutting out a test specimen 92 for
hardness measurement from the stainless-steel seamless belt. The test
specimen was prepared by cutting the seamless belt into a width of 20 mm
at a position 20 mm from the base portion side (ironing process starting
side) and then cutting-open the belt in the direction perpendicular to
the peripheral direction thereby providing a strip-like test specimen
with a length of 45 mm. Then, the test specimen was bonded to a glass
plate with an adhesive, Bond Aron AlphaPro No. 1 (product name of Konishi
Co., Ltd.) such that an inner surface side scraping against the ceramic
heater acts as a measurement surface. Subsequently, the test specimen was
left for two days until completely dried. Then, a wrapping film
corresponding to about 3 microns was used to polish the test specimen to
such a state that the surface roughness of the measurement surface does
not affect the hardness measurement.

[0092] As the measuring instrument, "FISCHERSCOPE HM2000" (product name)
manufactured by Fischer Instruments K.K. was used. The load was applied
so as to reach a measuring load of 500 mN in 20 sec from the time when an
indenter contacted the test specimen and the state was held for 5 sec. At
this time, the penetration amount of the indenter was about 2.5 μm.

[0093] The bending endurance test results and the hardness measurement
results before and after the stainless-steel seamless belt obtained in
the first embodiment was subjected to low temperature annealing are
listed in Tables 1 and 2. The low temperature annealing increased the
Vickers hardness by about 10% and the bending durability by about 110%.

[0094] Note that in the bending endurance test, for the all the test
specimens, a crack occurred in a position of the test specimen whose
bending portion had a maximum curvature.

Second Embodiment

[0095] The second embodiment will focus on the manufacturing method after
ironing and before cutting the metal cup-shaped member manufactured in
the second stage of the first embodiment, namely, the method of
subjecting the thin-walled bottomed cylindrical member 2 to a plastic
forming process without changing the length in the direction
perpendicular to the peripheral direction thereof.

[0096] In the same manner as in the first embodiment, a stainless-steel
metal flat plate is subjected to a deep-drawing process to obtain a
first-stage metal tube. Then, the first-stage metal tube is subjected to
an ironing process to obtain a bottomed cylindrical member 2. The
manufacturing method up to this has been described in detail in the first
embodiment and thus the description is omitted here.

[0097] Next, a hydraulic bulge forming process is used as the diameter
expansion unit to subject the bottomed cylindrical member 2 to a plastic
forming process. FIGS. 7 and 7B illustrate a method of subjecting the
bottomed cylindrical member 2 to a plastic forming process without
changing the length in a direction perpendicular to the peripheral
direction thereof. FIG. 7A illustrates a state in which the bottomed
cylindrical member 2 is inserted into a bulge die. A slight pressure is
applied to the bottomed cylindrical member 2 in a direction indicated by
an arrow so as to allow the base portion thereof to surely contact the
upper surface of a bulge inner die 21. Then, an internal pressure load is
applied for diameter expansion as illustrated in FIG. 7B.

[0098] Conventionally, a shrinkage amount H occurs in the seamless belt as
described in FIGS. 4A and 4B, but in the present embodiment, no shrinkage
amount H occurs because the base portion of the bottomed cylindrical
member 2 is clamped on the upper surface of the bulge inner die 21 to be
fixed.

[0099] Although not illustrated in FIGS. 7A and 7B, the binding member 11
used in the first embodiment is mounted on the side of an opening end
portion opposite to the base portion of the bottomed cylindrical member
2. In the same manner as in the first embodiment, a frictional force
between the rubber tube and the binding member 11 causes the side of the
opening end portion opposite to the base portion of the bottomed
cylindrical member 2 to be fixed. Thus, the base portion of the bottomed
cylindrical member 2 and the side of opening end portion opposite thereto
are fixed, which enables diameter expansion without changing the length
in the direction perpendicular to the peripheral direction of the
bottomed cylindrical member 2.

[0100] Subsequently, in the same manner as in the first embodiment, the
base portion of the bottomed cylindrical member 2 subjecting to the
diameter expansion is cut and finally is subjected to low temperature
annealing.

[0101] As described hereinbefore, the bottomed cylindrical member 2 with
remaining base portion is used for diameter expansion, which enables the
end portion on the base portion side of the cylindrical member 2 to be
fixed even in a state in which the binding member 11 is not mounted on
the bulge outer die 10 on the base portion side of the bottomed
cylindrical member 2.

[0102] The first embodiment of mounting the binding members 11 on both
sides has a disadvantage in that each time the seamless belt is inserted
and discharged, at least one of the binding members needs to be removed
from the bulge outer die 10, while the present embodiment has an
advantage of eliminating such a cumbersome work.

[0103] The evaluation results of the seamless belt obtained in this manner
are listed in Tables 1 and 2. The low temperature annealing has a great
effect in that Vickers hardness increases by about 10% and the bending
durability increased by about 100%.

Comparative Example 1

[0104] A stainless-steel seamless belt was obtained in the same manner as
in the first embodiment except that after the ironing process, the low
temperature annealing was carried out without performing the diameter
expansion in the peripheral direction. The evaluation results of the
seamless belt obtained in this manner are shown in Tables 1 and 2. The
low temperature annealing has no great effect as compared with that in
the first and second embodiments because the Vickers hardness increases
by about 10% and the bending durability increases by about 10%.

Comparative Example 2

[0105] A stainless-steel seamless belt was obtained in the same manner as
in the first embodiment except that the conventional hydraulic bulge
forming method was used as a means for performing the diameter expansion
in the peripheral direction. Note that the shrinkage amount H of the
seamless belt occurring in the hydraulic bulge forming in the comparative
example 2 was 0.3 mm on both ends.

[0106] The evaluation results of the seamless belt obtained in this manner
are shown in Table 1. The low temperature annealing does not have so
great effect as compared with that in the first and second embodiments
because the Vickers hardness increases by about 10% and the bending
durability increases by about 50%.

[0107] It is understood from the results in Table 1 that the number of
times of bending until a crack is found greatly increased in the present
embodiments than in the comparative examples in comparison between before
and after the low temperature annealing.

[0108] Thus, as described hereinbefore, the metal tube subjecting to the
ironing process is subjected to the plastic forming process for diameter
expansion without changing the length in a direction perpendicular to the
peripheral direction, followed by the low temperature annealing process,
which can provide a metal seamless belt having highly durable bending
fatigue characteristics while maintaining high hardness.

[0109] While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all such modifications and equivalent structures and functions.

[0110] This application claims the benefit of Japanese Patent Application
No. 2010-161411, filed Jul. 16, 2010, and Japanese Patent Application No.
2011-151979, filed Jul. 8, 2011, which are hereby incorporated by
reference herein in their entirety.